All observations were reduced within the ftools environment
[1]}. The XRT data analysis was performed using the
xrtdas standard pipeline package (xrtpipeline v. 0.13.4) in
order to produce screened event files. All X–ray data were acquired in
photon counting (PC) mode [2]} adopting the standard
grade filtering (0-12 for PC) according to the XRT nomenclature. For each
source, scientific data were extracted from the images using an extraction
radius of 47\(^{\prime \prime }\)  (20 pixels) centered at the optical coordinates of the
source, while the corresponding background was evaluated in a source-free
region of radius 94\(^{\prime \prime }\)  (40 pixels) within the same XRT acquisition. In all
cases, no emission above a signal-to-noise threshold \(S/N=3\)  was detected.
The XRT count rate upper limits in the 0.3–10 keV range were then measured
within the xspec package.
